It is known in the art of controlling permanent magnet synchronous machines (PMSM) that in order to start the machine reliably, the rotor position has to be known to some extent. If the position of the rotor is not known, the shaft of the motor may turn to the wrong direction or the shaft may begin to vibrate when starting the motor. Further, the starting of the motor may fail completely and the frequency converter may begin to magnetize a motor while it is in a stand-still state. In some processes or tools using the motor, the rotation of the shaft to the wrong direction may be detrimental to the process or the tool.
For reliably starting the PMSM, the rotor position has to be measured, estimated or forced to a known position before starting the motor. In simple and low-cost applications, the measurement of the rotor position is avoided due to the costs relating to measurement sensors and their maintenance.
Methods of estimating the rotor position are based on specific properties of the machine. These properties include the magnetic asymmetry of the rotor and magnetic saturation. The estimation methods are not generally suitable for different kinds of PMSMs due to differences in the above properties between different machines.
The position of the rotor can be simply forced to a specific position by applying a DC current to the stator of the machine. The forcing of the position is a simple and reliable way of ensuring the starting of the machine from a known rotor position. This procedure can be suitable in cases where the rotation of the rotor to the wrong direction cannot be allowed.
It is known to start the PMSM by feeding a current to the stator and increasing slowly the frequency of the current vector. This method of forced current vector rotation has to be taken from the use at higher speeds so to increase the performance of the drive. However, the method of forced current vector rotation is not suitable for all types of PMSMs and for all types of loads connected to the machine.